With a prevalence of information processing apparatuses such as a personal computer, a printing apparatus as am image generation terminal is also widely prevalent. In particular, an ink-jet printing apparatus which ejects ink from ejection openings onto the printing medium such as a paper to perform printing has various advantages, such as a non-impact and low noise printing system, a high density and a high speed printing operation, and easy application for color printing. From these points, the ink-jet printing apparatus is becoming a mainstream one in the field of a printing apparatus for personal use.
Such wide use of the ink-jet printing technology has required further improvement in print image quality. Particularly, since there is recently an environment where photos can be printed at home with ease by a print system, a quality of a print image not less degraded than in a silver halide photography has been demanded. In comparison with such sliver halide photography, a granular feeling in a print image is one of the conventional problems. On the other hand, various configurations of the print system for reducing the granular feeling are proposed.
For example, there is known an ink-jet printing apparatus using normal ink of cyan, magenta, yellow and black and further, ink of light cyan and light magenta which are lower in concentration of a color material such as dyestuff than the normal ink. Such an apparatus reduces a granular feeling by using ink of light cyan and light magenta in an area where the print density is low. In addition, in an area of high density, use of ink of cyan and magenta having a normal density realizes a wider color reproduction range and smooth gradation sequence properties.
Also, there is known a method of designing a size of a dot formed in the printing medium to be made small for reducing a granular feeling. For realizing this, there has been advancing the technology of reducing an amount of an ink droplet ejected from an ejection opening of the printing head. In this case, in addition to making an amount of an ink droplet be small, arranging many ejection openings in high density causes a high resolution image to be simultaneously obtained without impairing printing speeds.
Besides the aforementioned granular feeling reducing technology of focusing attention on the ink to be used, the technology of focusing attention on an area coverage modulation method is known as that of reducing a granular feeling by means of image processing. An ink-jet printing apparatus determines execution/non-execution of dot formation to each pixel and carries out printing according to the determination. In this processing, the multi-valued image data having density information is subjected to a quantization process to be finally converted into binary data, that is, data for determining execution/non-execution of dot formation. The print image of an area having the extent which is macroscopically observed, the density or the gradation is expressed by the number and the arrangement of printed dots. Such expression of density or gradation is generally called as an area coverage modulation method. The area coverage modulation method includes various dot arrangements for expressing the same density. For example, there is known a dot arrangement according to an error diffusion method as described in a paper by R. Floid and L. Steinberg: “Adaptive Algorithm for Spatial Grey Scale”, SDI Int'l. Sym. Digest of Tech. Papers, paragraphs 36 to 37 (1975). In addition, as a method other than the error diffusion method, there is known a dot arrangement by an ordered dither method as disclosed in Japanese Patent No. 2622429 or Japanese Patent Laid-Open No. 2001-298617. These methods can create an image having a good visual perception in which an arrangement of formed dots is excellent in dispersion properties and low frequency components in a spatial frequency of the dot arrangement is few. In this manner, when dot data (binary data) is obtained using an error diffusion method or a dither method, a binarization process in which print quality is considered is performed.
A so-called serial type of the ink-jet printing apparatus widely employs a multi pass method. It should be noted that words “pass” and “scan” used hereinafter have the same meaning. In the multi pass printing, the dot data for a certain area which is obtained as described above is divided into data for each ink color and each pass and the division is performed generally by using masks.
FIG. 1 is a diagram for explaining the multi pass printing and schematically shows a printing head and dot patterns printed in a case of completing an image by four times of scans. In FIG. 1, P0001 denotes a printing head. Here, for simplifying its illustration and explanation, the printing head having sixteen ejection openings (hereinafter, also referred to as nozzle) is shown. The nozzle array is, as shown in FIG. 1, divided into four groups of a first to a fourth group, each including four nozzles for use. P0002 denotes a mask pattern where pixels of a mask which permits printing (print permitting pixel) corresponding to each nozzle are painted in black. The mask patterns corresponding to four nozzle groups are complementary with each other. When the four mask patterns are overlapped, all the pixels of 4×4 constitute the print permitting pixels. That is, four mask patterns is used to complete printing in all the areas of 4×4.
P0003 to P0006 denote arrangement patterns of formed dots and show the process in which an image is completed by executing plural times of printing scans. As shown in this pattern, in a multi pass printing, each printing scan forms dots based upon binary image data (dot data) generated with use of the mask patterns corresponding to nozzle groups respectively. In addition, each time the printing scan is completed, a printing medium is conveyed in an arrow direction by the width amount of one nozzle group. In this way, for areas corresponding to the width of respective nozzle groups in the printing medium, images of respective areas are completed by four times of printing scans.
According to the multi pass printing as described above, density unevenness due to a variation in an ejection direction or an amount of ink between plural nozzles possibly generated in the manufacturing process of a print head or to an error in paper conveying that is performed between printing scans can be hard to be observed.
It should be noted that in FIG. 1, the four-pass printing in which scanning the same image area is executed four times is shown, but the multi pass printing is not limited to this four-pass printing. A two-pass printing in which an image is completed by twice of printing scans, a three-pass printing in which an image is completed by three times of printing scans, or a five or more-pass printing in which an image is completed by five or more times of printing scans may be applied.
In the multi pass printing, a number of printed dots in each printing scan can be adjusted or an operation frequency of a nozzle for which a trouble is easy to occur can be reduced, by changing an arrangement of a print permitting pixel in a mask pattern. That is, the multi pass printing can have modes in accordance with various purposes other than elimination of the above described density unevenness or bandings.
As described above, according to the recent ink-jet printing system, it is possible to output a stable image with a high quality at a high speed by wide variety of ink, implementation of various multi pass printings, adoption of a preferable area coverage modulation method (binarization method) and the like.
However, a configuration where the dot data obtained in the binarization of a dither method is made to be a data for each scan of the multi-pass printing by using the mask pattern may cause a problem related to a memory capacity or processing. More specifically, there are required two kinds of pattern data composed of a dither pattern used for binarization and a mask pattern used for dividing the dot data into a data for each scan. In this case, the capacity of the memory for storing this pattern data may increase. Also, two processes of the binarization using the dither pattern and data division using the mask pattern are performed and then a processing load may increase.
Patent reference 1: Japanese Patent No. 2622429
Patent reference 2: Japanese Patent Laid-Open No. 2001-298617
Patent reference 3: Japanese Patent Laid-Open No. 2006-050596
Non-patent reference 1: “Adaptive Algorithm for Spatial Grey Scale”, SDI Int'l. Sym. Digest of Tech. Papers, No. page 36-37, 1975